Ex girlfriend still seems semi interested, what to do?

So ya know how on Facebook, people the the "Like for a truth is..." stuff, well I did that with one of my ex girlfriends, and this is what she said:

"Truth is,

I MISS YOU! We used to date and that time was great. Every time I listen to "Risen Today" I think of you :D we need to hang when you come back to lake Geneva c:"

The thing is though is that she's like 16, and I'm 20. I dated her for a bit about 2 years ago, when I was a senior in high school, and she was a freshman, I was 18 and she was 14. it didn't last long, but it was a good relationship. I especially liked it because she is tiny like me (we are both close to 100 lbs), and we did make a cute couple.

But I just don't know what to do because I'm an adult and she's still a "kid" and I know all about the legal side of the law and everything, but I just don't know what to do.

Also I'm in college about 3 hours away from her, and I'm not sure what she wants to do after she graduates from high school.

Thanks for your help.

Updates:

Also, she does currently have a boyfriend, and I've been single for like 2 years now, she was my most recent relationship.